Audio volume control circuit



1942. w. VAN B. ROBERTS 2,273,143

AUDIO VOLUME common CIRCUIT Filed my 5, 1940 2 Sheets-Sheet 1 Fagl i v 2I a m f 1 7'0 Fallow/74 8745a 10 7'0 flex? Sfgge Souree 3nbentorFlall'er vanlilfoberlis v Gttorneg 1942- w. VAN B. ROBERTS 2, ,1

. AUDIO VOLUME CONTROL CIRCUIT v Filed July 5, 1940 2 Sheets-Sheet 2 vTo NezfJ/gge Wall2er mz z f-iliober v 7%? M v attorney Patented Feb. 17,1942 AUDIO VOLUME CONTROL CIRCUIT Walter van B. Roberts, Princeton, N.J., asslgnor to Radio Corporation of America, a corporation of DelawareApplication July 5, 1940, Serial No. 343,929:

4 Claims. (Cl. 179-171) My present invention relates to volume controlcircuits for resistance-coupled audio frequency amplifiers, and moreespecially to circuits which employ a variation of degenerative audio.feedback for volume control.

' The use of degenerative audio voltage feedback for volume control isknown. However, it has sufiered the disadvantage in .the past thatsuflicient control has not been obtained. Hence,

it may be stated that one of the important ob-- jects of my presentinvention is to provide a circuit wherein reduction of gain isaccomplished at first chiefly by an increase in the amount ofdegeneration employed, and subsequently chiefly by a reduction in theproportion of the output voltage of one tube that is applied to theinput electrode of the following tube.

Another object of this invention is to provide a composite volumecontrol of the aforementioned type with only a single adjustable elementto produce both the aforesaid actions.

Still otehr objects of my invention are to improve generally thesimplicity and efliciency of audio volume control circuits, and moreparticularly to provide volume control cricuits of input electrode 2 isconnected to one side of an audio voltage input network through anegative biasing battery 3 and an audio coupling condenser 4. The gridside of condenser 4 is connected to ground through a-resistor 5.

The cathodefi is connected toa slidable tap I through a self-biasingresistor 8, the latter be-.

ing appropriately by-passed for audio frequency currents by condenser 9.The tap I is the manthe degenerative feedback type which are not onlyreliable in operation, but which are economically manufactured andassembled.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill besLbe understood by reference to the following "description takenin connection with the drawings in which. I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings:

F'igures 1 to 6, inclusive, show respectively different embodiments ofthe invention.

-Referring now to the accompanying drawings, wherein like referencecharacters in the different figures designate similar circuit elements,in Fig. 1 there is shown that portion of an audio frequency amplifiercircuit which is essential to a proper understanding of this invention.It is to be understood that the invention is not restricted solely tothe audio frequency range, but may indeed be utilized in connection withsignal transmission tubes operating in frequency ranges higher than theaudio frequency range. Let it be assumed that the numeral I designatesan audio amplifier tube; of any well known type, whose ually adjustablevolume control device which is slidable along the resistor 5 in order tovary the audio voltage output of the amplifier system. The plate Ill isconnected to a source of positive voltage through a resistor II, and theaudio coupling condenser I2 may transmit the amplified audio voltagedeveloped across resistor II to further audio utilization networks. Itis to be understood that the audio voltage source which is coupled tocondenser 4 may comprise a preceding audio amplifier, or it may comprisethe load impedance of a radio receiver detector circuit. Further, theutilization stages which follow the tube I may comprise one or moreaudio amplifier tubes, or an audio reproducer may terminate the network.

The cathode slider I varies the amount of unby-passed resistance in thecathode circuit of tube I, and'thus exercises a control over thedegenerative feedback from the output to the input circuit of tube I. Itwill be appreciated that the portion of the resistor '5 between tap Iand ground is common to the input and output circuits of tube I, and,hence, audio voltage developed thereacross will be applied to grid 2 indegenerative phase. This results in a reduction in the audio voltageoutput of tube I. As the slider is moved upwards alongresistor 5 thedegenerative feedback of audio voltage is increased, since the sectionof resistor 5 between tap I and ground is increased in magnitude.Furthermore, the output voltage developed across resistor II becomes asmaller fraction of the total voltage developed in the entire outputcircuit of tube I, which includes not only resistor II but also theportion of 5 between slider I and ground. It will, therefore, b seenthat the tap I varies the audio voltage. output of tube I bysimultaneously controlling the degree of audio degenerative feedback andconcurrently varying the proportion of the audio output voltageimpressed upon coupling condenser I 2. This provides a simple andeffective method of controlling the volume of the audio amplifier. Biasis obtained for grid 2 by virtue of the direct current pressed on thefollowing audio tube.

audio input source is at ground potential.

voltage drop across resistor 8, and if desired, a bias source 3 may beutilized to replace, or augment, the bias resistor 8. The source 3 may,if necessary, be by-passed by condenser 3'.

In the arrangement of Fig. 2 the sliding contact I is prevented fromcarrying direct current. In this case the cathode resistor I3 is madesufficiently large to provide by itself a desired amount ofdegeneration. Resistor I3 is, 'however, effectively in parallel withrespect to alternating current with the portion of resistor which liesbetween the tap I and ground. Thus.

the degeneration is reduced as the slider I is moved downward. Thecondenser I4, arranged between slider l and the cathode end of resistorI3, is made very large if a flat frequency characteristic is desired. Ifcondenser I4 is not large,

then low audio frequencies will be degenerated more than the higheraudio frequencies. This effect is most pronounced at high gain settingsat slider I. That is, the effect will be most pronounced when the slideris adjusted well down on resistor 5.

Of course, this effect is sometimes desirable as it is well to make someautomatic reduction of the low audio frequency gain relative to the gainat higher'audio frequencies as the listening level is raised. In thearrangement of Fi 2, if resistor I3 is large enough for considerabledegeneration, it may produce more direct current voltage thereacross,which acts as the bias for grid 2, than is necessary. This excessivebias may be reduced 15y utilizing the bias battery 3 with its positivepole toward grid 2. However, if desired, the lower end of resistor 5 maybe connected to a sufliciently positive point on the plate voltagesource instead of to ground. As in the case of the arrangement of Fig. lthe control is complete in that the stage gain is a maximum with sliderI at the bottom of resistor 5, while the stage output is a minimum whenslider I is at the top of resistor 5.

In the modification shown in Fig. 3 the volume control device comprisesa double slider which simultaneously varies the degenerative audiofeedback, and the fraction of the total of the audio, output voltagedeveloped which is im- Hence, the grid 2 of tube l is coupled to thehigh potential side of the audio voltage source by the couplingcondenser 4, while the low potential side of the The cathode 6 isconnected to ground through the self-biasing resistor 8 and a portion ofresistor I5 which is arranged between the grounded tap I6 and the lowerend of the cathode resistor 8. A grid lead resistor I6 connects the grid2 to the lower end of resistor 8, and thereby completes a bias circuitfor grid 2. The plate of tube I is connected to the positive terminal ofthe plate current source through a path whichincludes a portion ofresistor I1 and the slidable tap I8.

The plate current source is denoted by the numeral I9. It will beunderstood that taps I6 and I8 are adjustedsimultaneously and in thesame direction along resistors I5 and I1. It will be noted that thegrounded tap I6 is connected P across resistor 3|.

tutes the output resistance of the stage, is decreased. Hence, there isprovided in this circuit a common means for simultaneously varying theamplifier output volume by concurrently varying the degree ofdegeneration to the input circuit while varying the fraction of theamplified output voltage delivered to the following stage.

Fig. 4 shows an arrangement similar to that shown in Fig. 3, but withthe plate of the tube I connected to the positive voltage source throughthe resistor 20, while the control resistor I5 is connected between theplate end of resistor 20 and the lower end of resistor 8. The single tap2| is slidable along resistor I5, the fixed end of the tap being atground potential. In this arrangement there is secured the same actionas in I the case of Fig. 3, but a single slider is sufficient.

wards the right the degeneration of the amplifier tube I is increasedand the audio voltage fed to the following stage is decreased. It is,also, possible to connect the right end of resistor I5 to the right siderather than to the left terminal of condenser I2, and thus utilize theportion of I5 to the right of slider 2| as the grid leak resistor of thegrid of the following audio stage. However, this introduces a tonecontrol effect unless condenser I2 is very large, this elfect beinggreatest when the gain is adjusted to small values.

In the arrangement of Fig. 5 the cathode of tube I is connected to theplate through the condenser 30 and resistor 3I, the grounded slidabletap 32 being adjustable along resistor 3I. The cathode of tube I is,also, connected to the plate through a second path which includes thebias resistor 33, the direct current source 34 and the resistor 35. Thenegative end of resistor 33 is grounded. The grid of tube I is connectedthrough the grid leak resistor I 6 to an intermediate point on thecurrent source 34. Inthe arrangement of Fig. 5 the cathode bias isconstant, and the grid of tube I is brought to a point on the platecurrent source 34 which is a littles less positive than the cathodepotential. The slider 32 is varied to control simultaneously thedegenerative audio feedback to the grid of tube I while varying theaudio output voltage developed The arrangement functions,

1 in so far as the senseof adjustment of slider 32 to the negativeterminal of plate current source I3. The resistor I1 is the output loadresistor of the audio amplifier, and audio voltage developed thereacrossis transmitted to the following audio stage through the couplingcondenser I2. As the sliders I6 and I8 are moved to the right themagnitude of the degeneration increases, since current at the slider 32.

is concerned, as the arrangement of Fig. 4. There is still,- however,some direct current at the slider 32.

In the arrangement of Fig. 6 there is no direct The bias for the grid oftube I is obtained by virtue of the resistance of the cathode choke 40which is arranged in' series between the cathode and the grounded end ofthe direct current source 34. Unless condenser I2 is very large, lowaudio frequencies are discriminated against when the gain setting ofslider 32 is low. On the other hand, at such settings, the degenerationat low audio frequencies is also discriminated against due to therelatively lower choke impedance. Hence, if condenser I2 is large theover-all result may be an accentuation of low audio frequencies at lowlistening levels, which is desirable.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention. as set forth in the appendedclaims.

What I claim is: 1. In an amplifier network, a pair of signal inputterminals, one of said input terminals being maintained at a constantpotential, a pair of signal output terminals, an amplifier tube, aconnection from one of said input terminals to the tube grid, aconnection from one of said output terminals to the tube plate, anunbypassed resistance between the tube cathode and a point of fixedpotential, a resistance between said plate and a point of fixedpotential, and a single means for simultaneously varying the magnitudesof said resistances in opposite senses whereby the ratio of the signalvoltage developed at said output terminals to the total signal voltagedeveloped in the circuit between said plate and cathode is reduced asthe amount of degeneration produced by said unbypassed resistance isincreased.

I 2. In an amplifier network as defined in claim l, wherein said tworesistances are theportions of a single resistance element, a sliderconnection therefor which is maintained ata fixed potential whereby thesum of said two resistances is maintained constant for all positions ofsaid varying means.

3. In an audio signal voltage transmission system, a tube provided withat least a cathode, a signal input electrode and an output electrode, apair of signal input terminals, one of said terminals} being at a fixedalternating potential,

terminal, at least a portion of said path being an unbypassed resistorin the space current path of .said tube whereby audio signal voltagedeveloped across the resistor is applied in degenerative phase to theinput electrode, a second resistive path connected between said outputelectrode and said one input terminal, an output circuit coupled to saidsecond path to utilize audio voltage thereacross, and means ,forsimultaneously adjusting'the relative magnitudes of said resistor andsecond path in opposite senses.

4. In an audio signal voltagextransmission system, a tube provided withat least a cathode, a signal input electrode and an output electrode, apair of signal input terminals, one of said terminals being at a fixedalternating potential, means connecting said signal grid to the secondof said input terminals, a resistive path connected between said cathodeand said one input terminal, at least a portion of said path being anunbypassed resistor in the space current path of said tube whereby audiosignal voltage developed across the resistor is applied in degenerativephase to the input electrode, a second resistive path connected betweensaid output electrode and said one input terminal, an output circuitcou-- pled to said second path to utilize audio voltage thereacross, andmeans, comprising a slidable tap in circuit with said one resistor, saidsecond path and said one input terminal, for simultaneously adjustingthe relative magnitudes of said resistor and second path in oppositesenses.

WALTER van B. ROBERTS.

